Polyolefins have been used extensively in a wide variety of applications including food packaging, textiles, and resin materials for various molded articles. Different polymer properties (melt flow, molecular weight distribution, density, etc.) may be desired depending on the intended use of the polymer. For example, polyolefins having relatively low molecular weights and narrow molecular weight distributions may be suitable for articles molded by an injection molding method. On the other hand, polyolefins having relatively high molecular weights and broad molecular weight distributions may be suitable for articles molded by blow molding or inflation molding. In other applications, medium-to-high molecular weight polyethylenes are desirable. Such polyethylenes have sufficient strength for applications which require such strength (e.g., pipe applications), and simultaneously possess good processing characteristics. Similarly, polyolefins having a particular flow index or within a particular flow index range are suitable for various applications.
Resins produced with chromium-based catalysts, such as reduced silyl chromate, generally have good extrusion properties. However, the resulting strength properties are typically less than resins produced with other catalysts. Similarly, resins produced from other catalysts may exhibit good strength, but may have poor extrusion properties.
Various mixed catalyst systems have heretofore been proposed to provide a blend of desirable resin properties, such as a mixture of Ziegler-Natta type catalysts and metallocene catalysts. Unfortunately, not all catalyst systems are compatible, as noted in U.S. 2006/0160965, which describes the necessity for careful transitions between incompatible Ziegler-Natta catalysts and chromium based catalysts, and uses a transition aid to scavenge co-catalyst from the Ziegler-Natta polymerization.
U.S. Pat. No. 5,330,950 and U.S. Pat. No. 5,408,015 disclose a mixed Ziegler-Natta/chromium catalyst system including a Ziegler-Natta catalyst including titanium and aluminum on a magnesium oxide support and a chromium oxide catalyst on a silica support. To maximize compatibility, the '950 patent suggests using a weight ratio of chromium catalyst to Ziegler-Natta catalyst in the range from 3:1 to 15:1; the '015 patent discloses a broader range, 6:1 to 100:1. It should be noted that these patents teach use of chromium catalysts as the majority catalyst component to maximize compatibility. Without being bound by theory, it is presently believed that the chromium catalyst was required to be the majority catalyst component because the Ziegler-Natta catalyst required a significant amount of an aluminum alkyl co-catalyst that negatively interacted with the chromium catalyst.
It would be advantageous to have a Ziegler-Natta catalyst capable of operating well without addition of high amounts of co-catalyst, thus allowing for its use with chromium based supported catalysts that would otherwise be overwhelmed by high amounts of co-catalyst.